This invention relates to surgically implantable joint prostheses and, more particularly, to a joint prosthesis that combines advantages of modular and nonmodular prostheses.
Artificial joints of the human body, including in particular knee and hip joints, have been available for 50 years or more and have been the subject of intense development for at least the last 20 years. The earliest designs provided metal-to-bone or metal-to-metal contact between the articulating surfaces of a joint. Friction and wear were significantly reduced in subsequent designs by the introduction of ultra-high molecular weight polyethylene (UHMWPE) as a load-bearing surface. For example, a typical knee joint prosthesis has a tibial component with a polyethylene load-bearing surface in contact with a metal femoral component. Early designs of this type had polyethylene cemented to the tibia, but it is conventional today to secure a polyethylene bearing to a metal base or tray which is anchored in the tibia, typically with the aid of a stem or peg extending into the medullary canal of the tibia.
Numerous methods of securing a polyethylene bearing to a metal base have been developed over the years, as exemplified by the following patents:
Patent No. Inventor Issue Date 4,205,400 Shen et al. June 3, 1980 4,217,666 Averill Aug. 19, 1980 4,501,031 McDaniel et al. Feb. 26 1985 4,714,474 Brooks et al. Dec. 22, 1987 4,795,468 Hodorek et al. Jan. 3, 1989 4,822,362 Walker et al. Apr. 18, 1989 4,938,769 Shaw Jul. 3, 1990 4,959,071 Brown et al. Sep. 25, 1990 4,963,152 Hofmann et al. Oct. 16, 1990 4,997,445 Hodorek Mar. 5, 1991 5,061,271 Van Zile Oct.29, 1991 5,080,675 Lawes et al. Jan. 14, 1992 5,108,442 Smith Apr. 28, 1992 5,330,534 Herrington et al. Jul. 19, 1994
There are two general types of components: modular and nonmodular. A nonmodular prosthesis has a bearing secured to the base during fabrication in the factory, typically by direct compression molding. A modular prosthesis has a prefabricated bearing designed to be attached to the base during surgery.
A modular prosthesis has several advantages over nonmodular prostheses, one of which is that an assortment of different prostheses, i.e., different base/bearing combinations, can be created in the operating room from a small inventory of separate bases and bearings of various sizes, shapes and other characteristics. With a modular prosthesis, an orthopedic surgeon can implant an appropriate base for the patient and then fit the patient with several trial bearings in the process of selecting an appropriate primary bearing to attach to the implanted base. Modular bearings are often readily removable, and in such cases they have the further advantage of facilitating revision surgery, which may become necessary in cases of traumatic injury or bearing surface wear, by enabling replacement of the bearing without removing the base.
There are also advantages to a nonmodular prosthesis construction, such as design simplicity due to the absence of a need for a retaining mechanism for a removable bearing, and relatively low cost. An even more significant advantage is that a nonmodular component is virtually immune to micromotion at the interface between the bearing and the base.
Micromotion is very difficult to avoid with modular components due to the need for clearances between bearing and base to ensure that they fit together during assembly in the operating room. A very secure locking method may avoid the problem, but nonmodular fabrication of the prosthesis, e.g., direct compression molding of a bearing onto a base, avoids the issue. Unfortunately, a nonmodular prosthesis has heretofore made revision surgery more difficult in that the entire prosthetic component must be removed and replaced. In addition to the extra operating time involved and extraction tools required, such as described in U.S. Pat. No. 4,459,985 to McKay, removal and replacement of the base requires sacrificing the existing fixation to the bone and has associated complications, including possible bone loss or fracture and the difficulty of reestablishing solid fixation. Nevertheless, it is conventional wisdom regarding a nonmodular prosthesis, as stated in the above-referenced Smith patent, that "the bearing component cannot be changed without changing the base component."